1. Field of the Invention
The present invention relates to a fuel injection control device for an internal combustion engine.
2. Description of Related Art
Japanese Patent Application Publication No. 2001-59441 (JP 2001-59441 A) discloses a fuel injection control device for an internal combustion engine (hereinafter a “conventional device”). This conventional device computes a fuel injection amount for realizing a target air-fuel ratio (hereinafter a “target injection amount”) by using an engine speed (a parameter value) that influences an intake air amount.
More specifically, when the engine speed is relatively low, a long time is required for a crank angle to be advanced by a predetermined angle. Thus, the engine speed may change significantly while the crank angle is advanced by the predetermined angle. When the engine speed changes significantly, just as described, this change has a significant influence on the intake air amount. Accordingly, in the case where the engine speed is sampled at relatively long intervals (for example, crank angle intervals of) 180° when the engine speed is relatively low, the significant change of the engine speed may not be reflected to the computation of the target injection amount. Consequently, the target air-fuel ratio may not be realized.
For this reason, in the conventional device, the engine speed is sampled at crank angle intervals of 10° when the engine speed is equal to a predetermined speed or lower.
On the other hand, when the engine speed is relatively high, a short time is required for the crank angle to be advanced by 180°. Thus, the engine speed hardly changes while the crank angle is advanced by 180°. Even when the engine speed changes, this change hardly influences the intake air amount. Accordingly, in the case where the engine speed is sampled at relatively short intervals (for example, crank angle intervals of 10°) when the engine speed is relatively high, the change of the engine speed that hardly influences the intake air amount may be reflected to the computation of the target injection amount. Consequently, the target air-fuel ratio may not be realized.
For this reason, in the conventional device, the engine speed is sampled at crank angle intervals of 180° when the engine speed is higher than the predetermined speed.
Just as described, in the conventional device, the intervals at which the engine speed is sampled are switched in accordance with the engine speed. Meanwhile, in the conventional device, a computation interval of the target injection amount is a constant interval (4 ms) regardless of the engine speed.
By the way, when multi-injection control, in which fuel is continuously injected for plural times within a short time period, is executed, parameter values (for example, the engine speed, an engine load, fuel pressure, and in-cylinder pressure) that are used to compute control values on fuel injection (for example, initiation timing of fuel injection and a fuel injection time period) may be changed due to an influence of the each fuel injection or the like during execution of plural times of the fuel injection. Accordingly, in order to realize the appropriate fuel injection in the each fuel injection, the control values are preferably calculated at timing that is the closest to initiation timing of the each fuel injection as possible, so that changes of the parameter values are reflected to the computation of the control values. However, in the conventional device, the computation intervals of the target injection amount, which is one of the control values, are constant. Thus, when each, of the computation intervals is set as a long interval, the changes of the parameter values may not be reflected to the computation of the control values on the each fuel injection. Therefore, the appropriate fuel injection may not be realized.
Furthermore, in the case where single-injection control, in which the fuel is injected only once within the short time period, is executed, even when the parameter values are changed due to the influence of the fuel injection or the like, there is no fuel injection to which these changes should immediately be reflected. Thus, even when the computation interval of the control value is longer than that in the multi-injection control, the appropriate fuel injection is sufficiently realized. In addition, if the computation interval of the control value is reduced even in such a case, a processing load on the computation is excessively increased. Thus, such shortening is not preferred. Meanwhile, as described above, the computation intervals of the target injection amount are constant in the conventional device. Accordingly, when each of these computation intervals is set as a short interval, the processing load on the computation may excessively be increased.